Fiber reinforced composites are rapidly emerging as a primary material for use in high performance applications such as manufacture of aircraft components. Fiber reinforced composites provide structural efficiency at lower densities compared to metallic structures, allowing for the manufacture of light weight, high strength components. Fiber reinforced composites may be prepared using a variety of techniques, for example, hand or automated layup of prepreg, filament winding, compression molding and resin transfer molding. Of these techniques, hand or automated layup of prepreg is most common.
A prepreg comprises a fiber reinforcement impregnated with an uncured or partially cured resin matrix. Prepregs are available in a variety of forms depending on the configuration of the fiber reinforcement. For example, when the fiber reinforcement comprises a fiber bundle (or tow), the prepreg is specifically referred to as a "towpreg". By way of another example, when the fiber reinforcement comprises a collimated series of fiber bundles, the prepreg is specifically referred to as "prepreg tape".
Prepregs are typically supplied to part fabricators who convert the material into cured composite components using heat and pressure to cure the resin. For example, when the prepreg is in the form of a tape, the part fabricator cuts lengths of the tape and places them on a tool surface in the desired ply orientation. This operation can be done manually or automatically and is generally referred to as "layup". When the tool has a complex or curved or vertical configuration, the prepreg preferably has good tack to hold the plies together and to the tool until layup is complete. The prepreg also preferably has good drape or conformability, allowing it to conform to the tool shape. Preferably, the prepreg cures uniformly to provide composite parts having high glass transition temperatures. This allows the cured composite to withstand a variety of stresses (such as elevated temperatures, mechanical stresses, exposure to solvents, etc.) without loss of structural integrity.
Epoxy resin compositions can be used as the resin matrix for prepregs. Several references describe epoxy resin compositions comprising fluorene amine curatives. U.S. Pat. No. 5,276,106 (Portelli et al.) describes a thermosettable epoxy resin composition prepared by dispersion of thermoplastic particles, curatives, hardeners, catalysts, and modifying additives into the epoxy resin at a temperature at which the thermosettable resin is liquid, generally at about 30.degree. C. to 60.degree. C. U.S. Pat. No. 4,684,678 (Schultz et al.) describes a thermally curable epoxy resin composition prepared by mixing aromatic polyepoxides and curing agent or agents and/or catalysts to form a substantially uniform mixture. WO 95/05411 (Hardy et al.) describes a thermally curable aromatic amine-epoxy composition prepared by combining a polyepoxy, a polyamine, and a cure accelerator with mixing until the solids are uniformly distributed.
While these compositions have proven useful for a variety of applications, a need still exists for an epoxy resin composition comprising fluorene amine curative that can be used to provide prepreg having tack, suitable viscosity characteristics even after aging, and which uniformly cures to provide cured composites that exhibit glass transition temperatures comparable to those of the corresponding cured neat resins (i.e., resins prepared without fiber reinforcement). Preferably, such an epoxy resin composition is solvent-free for environmental reasons and to preclude the presence of residuals which can cause porosity during cure, possibly resulting in reduced performance characteristics.